Bandwidth is a valuable resource in wired and wireless communication systems. Frequency may be reused in a wireless network in order to reduce cost. A signal occupying the same bandwidth as a desired signal, referred to herein as a co-channel signal, may cause interference and may severely limit the performance of a conventional single-user receiver. Moreover, a transmitted signal may travel along several paths to arrive at the receiver in a wireless environment. This multipath propagation may also give rise to signal fading and inter-symbol interference.
The effects of Co-Channel Interference (CCI) and Inter-Symbol Interference (ISI) can be reduced by using joint equalization/interference cancellation at a receiver. Among the various approaches, which include linear filtering and decision feedback, Joint Maximum likelihood Sequence Estimation (JMLSE) of co-channel signals may provide superior performance. See for example, Ranta et al, "Co-Channel Interference Cancelling, Receiver for TDMA Mobile Systems", IEEE ICC Proceedings, February 1995, pp. 17-21. However, Ranta's approach may only be applicable to stationary channels (stationary over a data burst). Unfortunately, radio signals in mobile communication systems generally undergo Doppler shift caused by vehicular motion. When data bursts are long, this generally results in time-varying signal fading. Yoshino et al. propose joint adaptive channel estimation and demodulation in Yoshino et al., "Interference Canceling Equalizer (ICE) for Mobile Radio Communication". IEEE Trans. Vehicular Tech., Vol. 46, No. 4, November 1997, pp. 849-861.
The receivers proposed in the above-mentioned publications obtain co-channel impulse response estimates jointly with the aid of training sequences of synchronized co-channel signals. In current Time Division Multiple Access (TDMA) cellular radiotelephone systems like GSM and D-AMPS, co-channel signals are not synchronized in either link. Thus, joint training may not be readily used for channel estimation. However, it is possible to exploit the training sequences of individual signals. One such technique which utilizes the training sequences of all signals is described in Wales, "Technique for Cochannel Interference Suppression in TDMA Mobile Radio Systems", IEEE Proc. Comm., Vol. 142, No. 2, April 1995. This technique is also generally applicable to stationary channels.
Giridhar et al. propose adaptive joint MLSE and MAPSD algorithms which can estimate channel responses blindly without requiring any training in Giridhar et al., "Nonlinear Techniques for the Joint Estimation of Cochannel Signals", IEEE Trans. Comm., Vol. 45, No. 4, April 1997, pp. 473-483. They use a technique called Per-Survivor Processing (PSP) which involves parallel channel state feedback in the recursion of an MLSE processor. PSP can provide an effective alternative to conventional adaptive algorithms that employ a single channel estimator. However, the acquisition performance of PSP-based MLSE receivers may have limitations as described in Chugg, "Acquisition Performance of Blind Sequence Detectors Using Per-Survivor Processing", IEEE VTC Proceedings, Vol. 2, May 1997, pp. 539-543.
In a TDMA system, each co-channel signal may be subject to an overall channel impulse response which comprises a transmit pulse-shaping filter, a dispersive medium and a receive filter matched to the transmit pulse-shaping filter. The transmit and receive filter responses are known a priori and can thus be used to constrain the overall channel impulse responses. In the case of asynchronous co-channel signals, ringing arising from sampling offsets may increase the number of channel taps to be modeled. Estimation error generally rises as the number of taps being estimated increases.